1. Field of the Invention
This invention relates to pneumatic radial tires and more particularly to a pneumatic radial tire for heavy load vehicles such as trucks, buses or the like including an improved bead portion reinforcing construction.
2. Description of the Prior Art
A pneumatic radial tire for heavy load vehicles comprises at least one carcass ply wound around a bead core from the inside toward the outside thereof to form a turn-up region and composed of radially or semi-radially arranged cords. A rubber stiffener is interposed between the carcass ply and the turn-up region thereof and is substantially triangular in section, the rubber stiffener extending from the bead core outwardly in the radial direction of the tire. A strengthening layer is arranged along the turn-up region of the carcass ply and composed of at least one metal cord layer, and a reinforcing layer surrounding both the strengthening layer and the turn-up region of the carcass ply is composed of at least two organic fiber cord layers. The cords of one layer cross with the cords of the other layer. The reinforcing layer extends continuously from a height which is higher than both the outer upper end of the strengthening layer in the radial direction and the upper end of the turn-up region of the carcass ply to the inner surface of the carcass ply.
The pneumatic radial tire constructed as above described exhibits a significantly excellent effect of preventing a separation failure that is induced at the outer upper end of the turn-up region of the carcass ply, but tends to induce a separation failure between the organic fiber reinforcing layer and the sidewall rubber layer. The use of techniques for improving the separation resistant property between the organic fiber reinforcing layer and the sidewall rubber layer results in degradation of the separation resistant property at the outer upper end of the turn-up region of the carcass ply. Then, the above two measures contradict each other.
Many attempts have been made to overcome such contradiction, but hitherto none has led to fully satisfactory results.